arm64: module: don't BUG when exceeding preallocated PLT count

When PLTs are emitted at relocation time, we really should not exceed
the number that we counted when parsing the relocation tables, and so
currently, we BUG() on this condition. However, even though this is a
clear bug in this particular piece of code, we can easily recover by
failing to load the module.

So instead, return 0 from module_emit_plt_entry() if this condition
occurs, which is not a valid kernel address, and can hence serve as
a flag value that makes the relocation routine bail out.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/kernel/module-plts.c

@@ -36,7 +36,8 @@ u64 module_emit_plt_entry(struct module *mod, void *loc, const Elf64_Rela *rela,
 		return (u64)&plt[i - 1];
 
 	pltsec->plt_num_entries++;
-	BUG_ON(pltsec->plt_num_entries > pltsec->plt_max_entries);
+	if (WARN_ON(pltsec->plt_num_entries > pltsec->plt_max_entries))
+		return 0;
 
 	return (u64)&plt[i];
 }

arch/arm64/kernel/module.c

@@ -386,6 +386,8 @@ int apply_relocate_add(Elf64_Shdr *sechdrs,
 			if (IS_ENABLED(CONFIG_ARM64_MODULE_PLTS) &&
 			    ovf == -ERANGE) {
 				val = module_emit_plt_entry(me, loc, &rel[i], sym);
+				if (!val)
+					return -ENOEXEC;
 				ovf = reloc_insn_imm(RELOC_OP_PREL, loc, val, 2,
 						     26, AARCH64_INSN_IMM_26);
 			}